Sertoli cell proliferation in neonatal boars is potentially androgen dependent. Hence, the immediate objective was to evaluate effects of androgen receptor-mediated signaling on the first wave of Sertoli cell proliferation. The experimental design employed littermate pairs of boars with one member assigned to receive a daily oral dose of flutamide, an androgen receptor antagonist, beginning at 1 wk of age and the littermate the canola oil vehicle. Experiment 1 examined the response at 6.5 wk of age after completion of the first wave of Sertoli cell proliferation, and experiment 2 examined the response at 11 wk of age after initiation of the second wave of Sertoli cell proliferation. Experiment 3 was designed to evaluate initial responses at 2, 3, or 4 wk of age. Additional littermates from four of the litters evaluated at 2 wk of age were hemicastrated at 8 days of age. Testis weight increased approximately 50% in the flutamide-treated boars compared with vehicle-treated littermates (P = 0.01) by 6.5 wk of age. Approximately 80% more Sertoli cells/testis were present in flutamide-treated boars at 6.5 wk of age compared with their vehicle-treated littermates (P < 0.01). Animals that were hemicastrated at 8 days of age had more Sertoli cells/testis than their intact littermates at 2 wk of age (P < 0.01), but flutamide inhibited the hemicastration response. Androgen receptor antagonism during postnatal Sertoli cell proliferation increases Sertoli cell numbers, as does hemicastration, but receptor antagonism initially inhibits Sertoli cell proliferation induced by hemicastration.